1. Field of the Invention
The field of art to which this invention pertains is hydrocarbon separation. Specifically this invention relates to an improved process which utilizes a crystalline aluminosilicate adsorbent and a desorbent to separate normal paraffins from a feed mixture containing normal paraffins. More specifically this invention relates to an improved normal paraffin separation process which employs a crystalline aluminosilicate adsorbent and a two-step desorption step to produce a high-purity normal paraffin product stream containing a reduced concentration of aromatic contaminants.
2. Description of the Prior Art
Applicant recognizes the abundance of prior art in the separation field especially that art relating to countercurrent fixed bed type operations which are commonly referred to as simulated countercurrent-flow fixed-bed type operations as particularly exemplified in U.S. Pat. No. 2,985,589.
Specific prior art patents which are considered closely related to the present invention are Broughton and Gerhold U.S. Pat. No. 2,985,589; Broughton U.S. Pat. No. 3,274,099; Pharis et al U.S. Pat. No. 3,732,325; Neuzil U.S. Pat. No. 3,696,107; Pharis et al U.S. Pat. No. 3,723,302; Adams et al U.S. Pat. No. 3,733,261; and Broughton U.S. Pat. No. 3,715,409. All of these patents relate to simulated countercurrent solid-fluid separation processes in which an extract component of a feed stream is separated by selective adsorption on a particular adsorbent and subsequently recovered as a product stream in a higher concentration than that in the feed stream. In each process there are various zones representing quantities of adsorbent material in which individual operations are taking place. In each, at least three operational zones are utilized: an adsorption zone, a purification zone and a desorption zone. In the adsorption zone, the selectively adsorbed extract material and perhaps some contaminant materials are adsorbed while the less selectively retained raffinate materials generally remain in the interstitial void spaces surrounding the adsorbent. The basic operation taking place in the purification zone is the purification of the adsorbed extract materials present in the adsorbent; the adsorbent in "passing" through the purification zone becomes more concentrated with the extract material and less concentrated with raffinate materials. In the desorption zone a desorbent material removes the adsorbed extract material from the adsorbent.
The first patent discloses the basic concept of a simulated countercurrent solid-fluid contacting process employing a fixed bed of solid adsorbent having moving input and output streams which allow a segregation of zones in which separate functions are taking place in order to separate a feed stream into a raffinate product component and an extract product component.
The second U.S. Pat. No. 3,274,099 includes the same basic processing steps as the first patent but also includes an additional input stream into the purification zone, which is located between the adsorption zone and the desorption zone. The input stream is a sweeping agent, a raffinate-type (that is, a material which is relatively unadsorbed by the adsorbent) compound having a boiling point to permit separation by distillation from the feed raffinate component, which is passed into the process to push raffinate material which is trapped in the interstitial void spaces between adsorbent particles in the purification zone back into an adsorption zone to prevent feed raffinate material from passing from the adsorption zone through the purification zone and into a desorption zone thereby contaminating an extract product with feed raffinate material. In one embodiment, the process of U.S. Pat. No. 3,274,099 is used to separate normal paraffins from isoparaffins.
U.S. Pat. No. 3,732,325 discloses a process which employs the same basic processing steps of the first patent and a particular adsorbent to separate aromatic hydrocarbons, particularly the C.sub.8 aromatics. In the process described in that patent a purification stream which comprises extract material is passed into the purification zone. The extract material can be taken either from an extract stream outlet from the process or from extract material which has been separated from desorbent material in an extract stream fractionator. The purification stream containing the extract material displaces from the interstitial void spaces between the adsorbent particles any raffinate materials carried into the purification zone, removes feed contaminants adsorbed by the adsorbent and reduces the quantity of desorbent which normally surrounds the adsorbent particles in the zone when no purification stream is used.
U.S. Pat. No. 3,696,107 discloses a process for separating para-xylene from a feed stream containing a mixture of C.sub.8 aromatics which employs the basic processing steps described in the first patent, a particular crystalline aluminosilicate adsorbent and a two-stage desorption operation in which a first desorbent stream contacts adsorbent in the desorption zone to effect the desorption of para-xylene from the adsorbent and a second desorbent stream contacts the adsorbent in the desorption zone to effect the pushing of desorbed para-xylenes from the interstitital void spaces between the adsorbent particles. One extract stream is withdrawn from the process.
In U.S. Pat. No. 3,723,302, which discloses a process for separating olefins from paraffins employing the basic processing steps described in the first patent and a particular adsorbent, a two-step desorption operation is again used. The process uses two desorbent materials both of which enter into the desorption zone. The first desorbent material contacts the adsorbent in the desorption zone and causes contaminants to be desorbed from the adsorbent while the second desorbent material is used to desorb the product olefins from the adsorbent contained in the same desorption zone. Two extract streams are withdrawn from the process, an extract contaminant outlet stream and an extract olefin outlet stream.
U.S. Pat. No. 3,733,261 also discloses a process for separating olefins from paraffins which employs the basic processing steps of the first patent mentioned. In that process one desorbent material is admitted in two places in the desorption zone and two extract streams are removed from the process, an extract contaminant stream containing aromatic contaminants and desorbent material and an extract olefin stream containing olefins and desorbent material.
U.S. Pat. No. 3,715,409 discloses a process for the separation of aromatic hydrocarbons which employs four zones and includes the steps of: passing an extract material input stream into the purification zone to effect the desorption and displacement of raffinate material; passing at least a portion of the raffinate output stream passing out of the adsorption zone into the buffer zone to effect desorption and displacement of desorbent material; and, passing a raffinate input stream into an adsorption zone to effect displacement of desorbent from the adsorbent in that zone.
In each of the processes described above an extract output stream and the raffinate output stream generally contain a desorbent material which must be removed from at least the extract output stream to produce a high purity extract product and to allow reuse of the desorbent material. In processes where it is the extract component and not the raffinate component that is the desired product, desorbent material is nonetheless usually separated from raffinate output stream so that the desorbent material can be reused in the process. Separation is typically done in fractionation means; at least a portion of the extract output stream and of the raffinate output stream are passed to respective fractionation means wherein desorbent material is separated to produce an extract product and a raffinate product essentially free of desorbent materials and a stream from each fractionation means containing one or more desorbent materials and a sweeping agent, if one is used in the process. In processes in which only one desorbent material and no sweeping agent is employed the streams can be combined with no further processing for reuse in the process. When more than one desorbent material or a desorbent material and a sweeping agent is used in the process these desorbent-containing streams must usually be further processed to separate the desorbent materials or a desorbent material from a sweeping agent. Prior to my invention this is done by passing at least one of the desorbent-containing streams from the extract-output-stream and raffinate-output-stream fractionation means to another fractionation means, a desorbent splitter, where the separation takes place to produce streams which can be recycled to different locations in the process.
The process of my invention relates to an improved process for separating normal paraffins from a feed stream containing normal paraffins and isoparaffins. Normal paraffins are used as raw materials to make a variety of products including straight-chain olefins and alcohols, proteins intended for animal or human consumption, and detergents. The improved process in one embodiment employs in combination a crystalline aluminosilicate adsorbent, a simulated moving-bed countercurrent processing scheme, a sweeping agent and a desorbent material to effect the separation of normal paraffins while another embodiment employs in combination the same adsorbent, processing scheme, sweeping agent and two desorbent materials to produce a normal paraffin product having a reduced concentration of aromatic contaminants. The improvement comprises a method of separating a desorbent material and a sweeping agent and recycling each for reuse in the process. The improvement permits a reduction in the size of the desorbent splitter and in the energy input required to operate the splitter over the size and energy input requirement for a splitter operated without the method.